In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, for example, an image is generally formed on a surface of a sheet (the term “sheet” is herein defined to include a paper sheet, a plastic film such as an OHP film and the like, and this definition is effective in the following description) through the following process steps.
First, a surface of a photoreceptor body having photoconductivity is evenly electrically charged and, in this state, exposed to light, whereby an electrostatic latent image corresponding to an image to be formed on the sheet is formed on the surface of the photoreceptor body (charging step and exposing step).
Then, a toner (minute color particles) preliminarily electrically charged at a predetermined potential is brought into contact with the surface of the photoreceptor body. Thus, the toner selectively adheres to the surface of the photoreceptor body according to the potential pattern of the electrostatic latent image, whereby the electrostatic latent image is developed into a toner image (developing step).
Subsequently, the toner image is transferred onto the surface of the sheet (transfer step), and fixed to the surface of the sheet (fixing step). Thus, the image is formed on the surface of the sheet.
In the transfer step, the toner image formed on the surface of the photoreceptor body may be directly transferred to the surface of the sheet, or may be once transferred to a surface of an image carrier (first transfer step) and then transferred to the surface of the sheet (second transfer step).
Further, toner remaining on the surface of the photoreceptor body after the transfer is removed (cleaning step). Thus, a series of process steps for the image formation are completed.
In the photoreceptor charging step, the developing step, the transfer step and the cleaning step out of the series of process steps, electrically conductive rollers are widely used, which each include a roller body produced from a rubber foam tube foamed and imparted with an electrically conductive property suitable for a use purpose thereof.
In order to produce the rubber foam tube for the roller body at lower costs at higher productivity, a continuous production method is preferably employed, for example, which includes the steps of extruding an electrically conductive rubber composition containing a foaming agent into a tubular body through a nozzle of an extruder, and continuously feeding the extruded tubular body in an elongated state without cutting to pass the tubular body through a microwave crosslinking device and a hot air crosslinking device of a continuous crosslinking apparatus to continuously foam and crosslink the rubber composition (see, for example, Patent Literatures 1 to 3).
For production of the electrically conductive roller, an electrically conductive rubber foam tube is produced, for example, from an electrically conductive rubber composition by the aforementioned production method, and cut to a predetermined length to form a roller body. Then, a shaft is inserted into a center through-hole of the roller body and fixed to the roller body, and the outer peripheral surface of the roller body is polished to be finished as having a predetermined outer diameter. Thus, the electrically conductive roller is produced, which includes the predetermined-diameter roller body and the shaft unified together.
Where the electrically conductive roller is produced by cutting and polishing the rubber foam tube formed through the foaming and the crosslinking under the conventional conditions disclosed in Patent Literatures 1 to 3 with the use of the continuous crosslinking apparatus, however, the outer diameter of the roller body of the electrically conductive roller is liable to gradually increase to significantly change after the production.
Therefore, even if the roller body is accurately finished so as to have a predetermined outer diameter in the polishing step in a production plant, for example, the roller body is often rejected due to an outer diameter dimensional error at customer's acceptance inspection. This may significantly reduce the production yield of the electrically conductive roller.
The change in outer diameter continues for at least about two weeks after the production. Even if the post-polishing outer diameter of the roller body is specified based on an expected dimensional change, the dimensional error occurs due to variation in the time of the customer's acceptance inspection. This also results in reduction in the production yield of the electrically conductive roller.
The outer diameter of the roller body of the electrically conductive roller should be strictly controlled in order to maintain the nip width and the nip pressure at constant levels for proper image formation, for example, when the electrically conductive roller is incorporated in an image forming apparatus and brought into press contact with the photoreceptor body or the like.